The farming of fish in the sea conventionally takes place in so-called open fish pens. An open fish pen includes a net forming a closed enclosure for the fish, allowing through flow. The fish pen is kept afloat with the aid of an encircling buoyancy system.
Open fish pens have some drawbacks. For the farming of salmon, it is a drawback that fish become infected with crustacean parasites, such as salmon louse, and with other diseases owing to viruses, bacteria and parasites. The fish escape, especially when tears are formed in the net by accident. An open fish pen discharges waste products such as feed not eaten and faecal matter into the recipient. The water temperature within the fish pen follows the temperature of the surrounding water. In the winter, the water temperature, especially near the surface, may be too low for the fish to grow optimally. In the summer, the water temperature may be too high for the fish to grow optimally.
A closed fish pen is an alternative to an open fish pen, and a closed fish pen solves some of the problems connected to an open fish pen. Closed farming fish pens are known within the art. They may be formed from a tight cloth material forming a bag. The patent publication NO160752/U.S. Pat. No. 4,711,199 discloses an example of such a fish pen. The fish pen is provided with a substantially flat bottom, the bottom being provided with a conically shaped sludge collector. The patent publication U.S. Pat. No. 4,798,168 discloses a cylinder-shaped fish pen in a material in cloth form with a cone-shaped bottom. Water is guided tangentially in at the surface of the fish pen, and an outlet has its inlet portion positioned in the water surface at the centre of the fish pen. Faecal matter and feed leftovers are collected in the lowermost portion of the bottom and pumped up to the surface via a pipe. A flexible suction pipe of a corrugated plastic pipe, for example, is attached to the inlet of the fish pen. At its free end, the flexible suction pipe is provided with a rope extending up to the surface. As the suction pipe is flexible, the free end may be lifted or lowered to the desired water depth. The length of the suction pipe is constant.
Closed fish pens formed from a rigid material are known as well. The patent publication NO 166511/EPO347489 discloses a semi-submersible floating platform which includes cylindrical silos provided with a cone-shaped lower portion. The patent publication NO 165783/U.S. Pat. No. 4,909,186 discloses a hull-shaped container for fish. The patent publication WO2010/016769 discloses a fish pen in a water-tight and substantially rigid material, the fish pen being substantially hemispherical. The patent publication states that the depth of a water inlet can be adjusted to have as optimum a water temperature as possible. The patent document WO2010/099590 discloses a fish pen composed of watertight fibreglass panels with an intermediate foam material to provide buoyancy. The fish pen is substantially cylinder-shaped with a flat bottom.
The paten publication NO 175341/WO9402005 discloses a closed fish pen with a cylindrical upper part and a conical lower part. The fish pen may consist of steel, concrete or a soft, reinforced plastic cloth. Water is pumped into the fish pen at the upper portion of the fish pen through a horizontal supply pipe. In its end portion inside the fish pen, the supply pipe is provided with a rotatable outlet to be able to direct inflowing water in a desired direction horizontally and vertically. In its lower portion, the conical lower part of the fish pen is provided with a funnel-shaped outlet. In a portion of the wall, the outlet is provided with a grid which holds back the fish, but which lets outflowing water pass. The outlet is further provided with static guide vanes to brake the approximately circular movement of the water in the fish pen into an approximately vertical downward movement.
Buoyancy systems for floating farming fish pens may consist of rectangular walkways in steel which are provided with floating bodies. The walkways are hinged together. The walkways form a grid of longitudinal and transverse walkways. Such a buoyancy system is usually used together with open fish pens. The seine, which forms the enclosure for fish, is placed in a square and secured to the encircling walkways on hooks projecting from separate poles or supports. The buoyancy system may also consist of at least one plastic pipe which is welded together into a ring. Usually, the buoyancy system consists of two concentric rings side by side, as shown in the patent publications WO 90/01872 and WO 91/17653, for example. Plastic fish pens having three concentric plastic rings are known as well. The plastic rings are connected by radially oriented clamps in plastic or steel. Walkways may be placed on top of two concentric rings. In an open fish pen, the seine is placed within the innermost pipe of the buoyancy system and is secured with projecting seine hooks. The seine hooks may be secured to the pipe or to a railing projecting up from the buoyancy system. The circumference for the seine in a plastic fish pen may be between 90 m and 160 m, for example, corresponding to a diameter of approximately between 30 m and 50 m.
Known closed fish pens solve some of the drawbacks connected to an open fish pen. The known closed fish pens have some drawbacks as well.
To ensure that the water exchange rate is sufficient to maintain a good water environment within the fish pen, it is usual to pump in so much water that the water surface within the fish pen is higher than the water surface outside the fish pen. Thereby the pressure inside the fish pen is greater than the surrounding pressure, and water will flow out of the fish pen through formed openings. This subjects the buoyancy system of a closed fish pen to greater forces than the buoyancy system of an open fish pen of the same size. In addition to keeping the actual cloth or net of the fish pen afloat, the buoyancy of the buoyancy system must be dimensioned for holding the amount of water inside the fish pen that is above the water surface of the surrounding water. This water constitutes a considerable mass. In addition, this water has a moment of inertia that makes the wave influence on the buoyancy system greater than in an open fish pen in which the wave motion passes the buoyancy system and into the fish pen substantially unobstructedly. As mentioned, the walls of closed fish pens may include a material in cloth form. Owing to the fact that the amount of water within the fish pen gives negative buoyancy, the material in cloth form must have great tear strength. A material in cloth form cannot be attached to the buoyancy system in the same way as a seine. A perforation of the material in cloth form will impair the tear strength. The hole may be reinforced with a grommet, but it is difficult to match the spacing of a plurality of holes to a plurality of hooks in such a way that the cloth is kept smooth. The water pressure inside the fish pen will stretch the cloth, while too short distances between the hooks will create folds. Too wide a spacing of the hooks in relation to that of the holes formed makes it impossible for one or more of the hooks to be used. This impairs the strength of the attachment of the fish-pen cloth to the buoyancy system.
Over time, a thin sludge will form inside a closed fish pen. The sludge consists of faecal matter and small feed particles. The feed particles are fatty. This sludge has poor settling properties. In fish pens with approximately flat bottoms, whether they have substantially cylindrical side walls or are ball-shaped, this sludge will lie along the bottom and only slowly creep in towards the centre of the fish pen. The outlet of the fish pen is usually positioned at the centre of the bottom. The sludge forms a pool which catches further faecal matter and feed leftovers and prevents them from being carried towards the outlet. The rate of the water flowing through may be increased to carry the sludge towards the outlet, but experience has shown that this may make the sludge dissolve and get mixed into the water within the fish pen again, especially if the water flow is somewhat turbulent and deviates from the desired laminar flow. When an oxygenation system is used, with a supply of gas bubbles near the outlet of the fish pen, the particles of the sludge will easily be carried all the way to the water surface within the fish pen. Sludge dissolved in the water creates cloudy water and gives bad water quality for the fish. Fish that die will sink towards the bottom of the fish pen and will start to rot there if not removed. Mortality may be due to disease caused by pathogenic organisms, and it is important to remove dead fish quickly in order to reduce any infection pressure on the fish in the fish pen. The patent publication NO332341/WO2011133045 discloses a closed fish pen with a cone-shaped bottom. In one embodiment, all the waste water is brought up to a sludge tank which is positioned on the floating collar of the fish pen. In an alternative embodiment, feed leftovers and faecal matter are partially separated from the water at the bottom of the fish pen. The waste water is carried up to the sludge tank, whereas feed leftovers and faecal matter are carried up to the surface in a separate outlet pipe. The patent publication NO175082/WO9323994 discloses an outlet for vessels on land. The vessels are arranged for farming fish and are provided with relatively flat bottoms. A primary water flow and a secondary water flow carry particles in the water in towards the centre of the vessel. At the centre of the vessel, the waste water moves out through a slot and into a circular particle trap formed as an annular space around the outlet pipe. The water flows out of a row of holes in the outlet pipe, whereas deposited material is moved by the water flow through a slot opening and down into a particle outlet. The patent publication NO318527/U.S. Pat. No. 6,443,100 discloses an outlet for a closed fish pen. An outlet pipe extends through the bottom portion of the fish pen and projects upwards in the fish pen. The water flows out of the fish pen through a slot in the upper portion of the outlet pipe. Deposited material sinks down along the outside of the outlet pipe and is carried out of the fish pen through an opening in the fish pen at the outlet-pipe lead-through in the fish pen.
Water will have to be pumped into a closed fish pen because the pressure inside the fish pen is greater than that on the outside. The inflowing water creates currents within the fish pen. The currents should be such that the entire water volume is replaced regularly. Ideally, the flow should be laminar and run evenly from the inlet to the outlet. The patent publication NO 160753 discloses an inlet for a floating tank or a land-based tank, in which the water is carried into the tank through a pipe through the wall of the tank. On the inside of the tank, the water is carried through a vertically oriented nozzle device with fixed nozzles that let the inflowing water have a tangential component. The vertical nozzle device may be positioned with a horizontal distance to the wall of the vessel. The patent publication WO 2006/000042, too, discloses a vertically oriented nozzle device on the inside of a floating tank. The inflowing water gets a tangential component. The water is carried into the tank through a pipe through the wall of the tank. The patent publication U.S. Pat. No. 5,762,024, too, discloses a vertically oriented nozzle device on the inside of a floating tank. The water is carried into the tank from above, and the nozzle openings are positioned in the lower portion of the nozzle device. The water gets a circular flow in the tank. The flow rate of the water is adjusted by changing the speed of the pump. The nozzle devices described in these three patent publications are fixed so that the direction of flow of the water out of the nozzle device is constant. The patent publication NO 332589 discloses a floating tank with a vertically oriented nozzle device as well. The nozzle device is preferably provided with directionally adjustable slots or nozzles. NO 332589 does not show how this is to be done in practice. NO 332589 also discloses that the nozzle device may consist of an outer pipe and an inner pipe which can be rotated, lifted and lowered relative to each other in a controlled manner. In this way, the amount, direction and level of the inflowing water may be adjusted. NO 332589 does not show in detail how this is to be done. NO 332589 also discloses water intake pipes arranged telescopically. The lengths of the pipes, and thereby the depth of the inlet opening, may be adjusted with a wire extending through the pipe and being attached to the lower portion of the pipe. The patent publication NO 327035 discloses a nozzle device for a land-based tank. The nozzle device includes a vertically oriented, fixed distributor pipe. The distributor pipe is provided with a number of nozzle openings along the longitudinal direction of the distributor pipe. The nozzle openings extend axially in a slit shape with a length exceeding the width by a factor of 2 at least. A damper member provided with holes can be displaced axially along the distributor pipe. The damper member is provided with a number of slit openings corresponding to the slit openings of the distributor pipe.
As mentioned in the foregoing, the water within a closed fish pen may have a water surface above the water surface of the surroundings of the fish pen. The water within the fish pen may have been pumped up from a water depth at which the salinity of the water is greater than the salinity of the water on the outside of the fish pen. The water within the fish pen thereby has a greater density than the surrounding water. It has turned out that on a loss of the electrical power driving the pumps supplying the fish pen with water, the water will first flow out of the fish pen, until the water surface within the fish pen is level with the water surface on the outside of the fish pen. If the water within the fish pen has a greater density than the surrounding water, the water will continue to flow out of the fish pen. The water on the outside of the fish pen may flow past the fish pen at a relatively great velocity. It has turned out that once a water flow out of the fish pen has been established, it may be maintained by the water that is flowing past on the outside. A closed fish pen that includes walls in a material in cloth form will be extended when the pressure within the fish pen is greater than the pressure on the outside of the fish pen. When the water begins to flow out of such a closed fish pen, the walls of the bag will start to collapse. This will happen especially in the lower portion of the fish pen while the upper portion of the fish pen is kept in shape by the buoyancy system of the fish pen. The fish pen will thereby get a reduced volume and this has a stressful effect on the fish in the fish pen.
The water may be carried out of a closed fish pen by an outlet in the bottom portion of the fish pen. The outlet opening must be secured with a grating or the like, to prevent fish in the fish pen from escaping from the fish pen through the outlet. Over time, such a grating will become covered in sludge. The grating will also become fouled with, for example, shells and tunicates. The inside of the closed fish pen will also be fouled. Such fouling can be removed by washing. This leads to relatively large amounts of material sinking down towards the outlet and settling on the grating. The grating may thus partially become clogged up with sludge, fouling and material from the wall of the fish pen. As the grating partially clogs up, the water flow resistance out of the outlet will become larger. This is counteracted by the water column over the grating becoming larger. This is to say, the level of the water surface within the fish pen rises relative to the water surface of the surrounding water. This in turn increases the load on the buoyancy system. As illustrative dimensions it may be indicated that a water-level increase of 1 cm within a closed fish pen may result in the buoyancy system being lowered 7-8 cm further down in the water.
A substantial amount of water will have to be pumped into a closed fish pen to maintain a good water environment for the fish. The water may advantageously be pumped up from a depth at which the water temperature is more constant than at the surface. This also has the advantage of the fish being exposed to smaller amounts of parasites as these are usually located in the upper water layers. A supply channel with a large diameter has the drawback of the underpressure in the supply channel possibly making the wall of the channel collapse. This can be counteracted by increasing the wall thickness. The pipe will thereby be more rigid. A rigid pipe has the drawback of water currents pressing harder against the pipe. In particular the attachment of the pipe to the pump housing and the attachment of the pump housing to the buoyancy system of the fish pen will thereby be subjected to an increased strain.